This invention relates to a car starting motor and more particularly, to a permanent magnet field type starting motor provided with an auxiliary pole in which an auxiliary pole and a permanent magnet are juxtaposed with each other.
It is known to juxtapose an auxiliary pole with, and close to, a permanent magnet for improving the torque characteristic of a permanent magnet field type d.c. motor, as disclosed in Japanese Patent Publication No. 35721/1973. This auxiliary pole is mounted on the magnetizing side of armature reaction, and consequently, to torque characteristic markedly changes with the pole are ratio and position of juxtaposition of the auxiliary pole. The pole arc ratio is expressed by either the ratio of the center angle .theta..sub.3 of the auxiliary pole to the center angle .theta..sub.2 of the field, i.e., .theta..sub.3 /.theta..sub.2, or the ratio of the center angle .theta..sub.3 of the auxiliary pole to the center angle .theta..sub.1 of one pole.
The abovementioned reference does not describe in detail the position and size of the auxiliary pole but merely states that the region affected by the demagnetizing action of the armature reaction is set to the width of the auxiliary pole. The reference neither stipulates the optimum size of the auxiliary pole, nor clarifies its effect.